CLEAVAGE. 47 



peripherally with the underlying yolk mass and consequently with each other. 

 The analogy between this condition and that described for the frog's egg is 

 complete with the one exception that in the latter the cleavage furrows cut 

 completely through the yolk cells or the yolk-containing portions of the cells, 

 while in the bird's egg the amount of yolk is so great that the cleavage furrow 

 merely passes a short distance into it without completely dividing it into seg- 

 ments. The fourth cleavage plane is tangential, cutting off the apices of the 

 six pyramidal segments. The germ disk after the fourth cleavage thus con- 

 sists of six small superficial central cells and six larger cells which surround 

 the small cells and also separate the latter from the underlying yolk. From 

 this point radial and tangential cleavages follow each other without any sem- 

 blance of regularity. The result is a mass of small cells lying at the center of 



y.s. g.d. s.c. w.y. 



FIG. 29. From a vertical section through the germ disk of a fresh-laid hen's egg. Duval, Hertwig 

 g.d., Upper layer of germ disk; s.c., segmentation cavity; w.y., white yolk (See Fig. 7); y.s., lower 

 layer of germ disk (yolk cells, merocytes). 



the disk and surrounded by larger cells (Fig. 28, c, d). The smaller cells are 

 completely separated from the underlying yolk while the larger cells are for a 

 time continuous with it (Fig. 29). 



Comparing the unequal holoblastic cleavage of the frog's egg with discoidal 

 meroblastic cleavage as seen in the eggs of Birds, it becomes immediately 

 evident that the differences between them are explainable entirely by reference 

 to the greater quantity of yolk in the bird's egg. The real activity of segmenta- 

 tion is in both cases confined almost wholly to the protoplasm. In the frog's 

 egg the amount of yolk present is sufficient to impede segmentation in the 

 larger cells but not to prevent it. In the bird's egg the amount of yolk is so 

 great that it cannot be made to undergo complete segmentation. 



Some General Features of Cleavage. 

 Cleavage in Mammals. 



The two fundamental laws of cleavage as formulated by Sachs are : 

 i. That each cell tends to divide into equal parts. 



